It is becoming a necessity to lighten motor vehicles because of the requirement to reduce CO2 emissions to 140 g/Km by 2008. This lightening may be achieved only by increasing the level of mechanical strength of steels in order to compensate for the reduction in sheet thickness. It is therefore necessary to increase the mechanical properties while reducing the thicknesses of the sheet with which the parts used are produced. This approach reaches its limits with the reduction in rigidity of the parts and the appearance of noise and vibrations unacceptable in the intended applications in the automobile field, where noise is a discomfort.